Corrosion-resistant coatings



United States Patent 3,397,092 CORROSION-RESISTANT COATINGS Walter R. 'Cavanagli, Detroit, Mich., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Feb. 27, 1964, Ser. No. 347,710 8 Claims. (Cl. 1486.16)

ABSTRACT OF THE DISCLOSURE A process for imparting improved corrosion resistance to case hardened ferrous metal surfaces wherein the case hardened ferrous metal surface is heated to a temperature within the range of about 300 to 750 degrees F. for 15 minutes to 3 hours and any oleaginous materials are removed from the metal surface. Thereafter, a protective phosphate coating is applied to the heat treated metal surface. The removal of oleaginous materials from the metal surface, prior to the application of the protective phosphate coating may be effected simultaneously with the heat treatment, by using a sufliciently high heating temperature, or as a separate cleaning step either prior or subsequent to the heat treatment.

This invention relates to the coating of ferrous surfaces to improve their resistance to corrosion and more particularly relates to an improved process for treating ferrous metal surfaces prior to the application of a corrosion resistant phosphate coating.

It is known that the resistance to corrosion of ferrous surfaces can be improved by forming on these surfaces protective coatings of metal phosphates, such as those which consist of mixtures of zinc and iron phosphates or manganese and iron phosphates. For the most part, the application of such corrosion resistant phosphate coatings to various types of ferrous surfaces has given excellent results, in terms of the resistance to corrosion which has been obtained. One type of surface, however, which has proven difficult to protect with phosphate coatings has been that of ferrous metal parts which have been case hardened, eg by carborization, nitridation, and the like. Although phosphate coatings of the above type have been applied to such case hardened ferrous surfaces, the corrosion resistance of these phosphate coated case hardened ferrous parts has not been completely satisfactory. Rusting and similar corrosion of such case hardened parts has frequently occurred, often in less than one hour, when tested without an oil finish in a 5 percent salt fog, operated in accordance with American Society for Testing Materials Method B-117.

It is, therefore, an object of the present invention to provide an improved process whereby case hardened ferrous metal surfaces can be treated to provide corrosion resistant phosphate coatings on their surfaces.

A further object of the present invention is to provide an improved treatment for case hardened ferrous metal surfaces, which treatment results in improved corrosion resistance of phosphate coatings subsequently applied to such surfaces.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

thereafter, applying to the surface a corrosion resistant phosphate coating. It has been found that ferrous metal surfaces which are treated in this manner have greatly improved corrosion resistance over similar surfaces which have merely been case hardened and then treated with the phosphate coating material.

More specifically in the practice of the present invention, the metal surfaces treated are ferrous metal surfaces. By ferrous metal surfaces it is meant to include surfaces of iron, steel, and alloys which contain a major amount of iron or steel. These ferrous metal surfaces which are to be treated are first case hardened. Many of the various case hardening techniques, as are known to those in the art, may be used. For example, the ferrous surfaces may be carburized, e.g., heated in a closed reaction vessel in the presence of finely divided carbon and then oil quenched, they may be nitridated, e.g., heated in an atmosphere of nitrogen or ammonia, and the like. It is believed that the details of these and other case hardening processes are suificiently well known to those in the art that further description of them is not necessary.

It is to be understood that neither the exact nature of the case hardened surface nor the manner in which case hardening was effected are essential in the practice of the present invention. It is only important that the ferrous metal surface to be treated has been case hardened. In this respect, it is to be appreciated that the case hardening treatment may be carried out immediately preceding the remaining steps of the present process. Alternatively, of course, the case hardening of the ferrous metal surfaces may be carried out as a separate step, appreciably in advance of the remainder of the process. For example, the ferrous metal materials, after case hardening, may be stored for extended periods of time, e.g., six months or more, before being further processed in accordance with the present invention. Thus, it is to be understood that the present invention includes the processes set forth hereinabove for treating ferrous metal surfaces wherein case hardening is the initial step of the process, as well as a process wherein the steps of removal of oleaginous materials, heating, and application of phosphate coating are carried out on a ferrous surface which has previously been case hardened.

After case hardening of the ferrous metal surface to be treated has been effected, the case hardened surface is heated at a temperature within the range of about 300 to about 750 degrees F. for a period of time from about 15 minutes to about 3 hours. It will be appreciated, in general, that at the lower temperatures, the longer heating times will be used and vice versa. For example, where the case hardened surfaces are heated at a temperature of about 350 degrees F., heating times of two to two and one-half hours are typical, while at temperatures of 600 degrees F., heating times of 30 minutes to 1 hour may be used.

It is to be understood that insofar as obtaining improved corrosion resistance is concerned, the above temperatures and times are merely examplary of those which may be used, with both lower and higher temperatures as well as longer and shorter heating times producing comparable results. It is only insofar as any adverse effects on the case hardened surface are concerned that the above times and temperatures become important. For example, it has been found that when the temperature exceeds about 400 degrees F. and/or the time exceeds about 1 hour, there may be an adverse efliect on the case hardness of the ferrous metal surface. Accordingly, it is preferred that the heating step of the present process be carried out at temperatures within the range of 300 to about 750 degrees F. for a period of time from about 15 minutes to about 3 hours, in order to minimize degrading of the case hardened surface. It will be appreciated, however, that processes utilizing heat treatment steps at times and temperatures greater than the above preferred ranges, including those wherein there is a degradation of the case hardened ferrous surface, which result in improved corrosion resistant properties are deemed to be equivalent to those processes which are carried out within the above preferred ranges of time and temperature and hence are deemed to be included within the scope of the present invention.

One important feature of the present process is that, following the case hardening and prior to the application of the phosphate coating, the surfaces be free of oleaginous materials. By oleaginous materials, is meant various materials of an oily and/or greasy nature which may be on the ferrous metal surface as a result of handling, prior treatments, e.g., oil quenching in the case hardening treatment and the like.

Preferably, these oleaginous materials are removed from the case hardened ferrous metal surface either prior to or during the heat treating step of the present process. It will be appreciated that the order of the removal step in relation to other steps may be changed, provided equivalent end results are obtained. Similarly, it is to be understood that removal of these materials may have been effected long prior to the heating step, as a separate operation, in the same manner as the case hardening, as has been described hereinabove.

Removal of these materials may be carried out before the heat treating process, utilizing various materials for cleaning the oleaginous materials from case hardened ferrous metal surfaces, including alkaline type cleaners, acid type cleaners, as well as solvent cleaners. These may be of many and diverse types as are known to those in the art, such as alkaline cleaners containing caustic soda, soda ash, alkali metal phosphates, and the like, in combination with various of the surface active materials, including the anionic, nonionic, and cationic surfactants. Similarly, various acid cleaners, including those containing sulfuric acid, hydrochloric acid, phosphoric acid, and the like may also be used. Additionally, various solvent cleaning compositions, such as those used in solvent vapor degreasers, including trichloroethylene, perchloroethylene, methylene chloride, carbon tetrachloride, and the like, may also be used. It is generally preferred that the oleaginous materials be removed by such cleaning compositions prior to the heating step, the baking and hardening effect of the heat treatment on these oleaginous materials frequently making them more diflicult to remove after the heat treatment. Accordingly, it will be understood that the removal of the oleaginous materials from the ease hardened ferrous metal surface may be effected preferably before the heat treatment step as above or during this step by utilizing sufficiently high temperatures and sufficiently long heating times to effectly burn or oxidize the oleaginous materials from the surface. Typical temperatures and times are those in excess of about 550 degrees F. and 30 minutes.

It will be appreciated that various means may be used to effect heat treatment of the case hardened ferrous metal surfaces. For example, either direct or indirect heating means may be utilized, such as gas-fired ovens, or the like. Additionally, electrically heated tempering ovens may also be used.

To the heat treated or tempered case hardened ferrous metal surface, which is free from oleaginous materials, is applied a corrosion resistant phosphate coating. As has been indicated hereinabove, these coatings are preferably of the heavy phosphate coating type containing phosphates of metals, such as iron and zinc or manganese phosphate. The coating solutions utilized are generally solutions of acid metal phosphates, e.g. the zinc and manganese phosphates, and are well known to those in the art. Typical of the protective phosphate coating solutions which may be used are those as described in U.S.

4 Patents 1,911,726, 2,001,754, and a copending application Ser. No. 237,340, now U.S. Patent No. 3,268,367.

These and other suitable heavy protective phosphate coatings may be applied to the heat treated, case hardened ferrous metal surfaces by many convenient means, as for example, by immersing the hardened and tempered parts, free from oleaginous materials in the hot phosphatizing solution. The parts may be suspended in the phosphatizing solution, placed in mesh type baskets and immersed or placed in barrels which are immersed in the phosphatizing solution and then rotated. The details of these and other techniques are known to those in the art so that further description of them is not necessary. Once the application of the corrosion resistant phosphate coating has been completed, the coating may be rinsed and dried. Additionally, if desired, the coating may be given further, ancillary treatments, as are known to those in the art. Typically, the parts are first rinsed in water to remove any soluble phosphate salts and then dipped in chromate solutions, such as those described in U.S. Patent 2,970,935, prior to drying. Additionally, if desired, the phosphate coating may be dipped in or sprayed with oil to provide additional corrosion resistance.

In order that those skilledin the art may better understand the method of the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, the temperatures are in degrees Fahrenheit.

EXAMPLE 1 Steel bolts which have been case hardened by gas carburizing Were cleaned in trichloroethylene vapors and then heated for one hour at 600 degrees F. Following the heating step, the parts which were substantially free of oil and similar oleaginous materials, were coated with a zinc phosphate solution as described in U.S. patent application Ser. No. 237,340, now U.S. Patent No. 3,268,- 367. The parts were then rinsed in cold water and given a chromate rinse as described in U.S. Patent 2,970,935. The thus-treated parts were then dried and subjected to a 5 percent salt spray test for a period of sixteen hours. Upon removing the test pieces from the salt spray at the end of this time, there was substantially no rust evident on any of the pieces subjected to the test. When these parts were rated according to ASTM Method D-610-43, they were rated 9.5 (10 equals no rust and 0 equals percent rust).

By way of comparison, the above procedure was repeated with the exception that the case hardened steel bolts were not heated between the vapor degreasing and the application of the zinc phosphate coating. These test bolts were also subjected to the 5 percent salt spray test for a period of sixteen hours. At the end of this time, upon removing the bolts from the salt spray, it was noted that they were very severely rusted, and were given a ratin of 5, intermediate between no rusting (10 rating) and complete rusting (0 rating).

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that steel test bolts, which had not previously been case hardened, were heated in an ammonia atmosphere to effect case hardening thereof by nitridation. The thus case-hardened bolts were cooled by quenching in oil and thereafter were heated for about one hour at 600 degrees F. Following the heating, the bolts, which were substantially free of oil and similar oleaginous materials, were then coated with the phosphate material as in the previous example. Upon subjecting these thus-treated test pieces to the 5 percent salt spray test for a period of sixteen hours, it was found that there was substantially no evidence of rusting or other corrosion on the test pieces and they were rated as 8.3.

By way of comparison, the above procedure was repeated except that the heat treatment was omitted. After subjecting these parts to the 5 percent salt spray for 16 hours, they were rated as 5.3 according to ASTM Method D-610-43.

EXAMPLE 3 The procedure of Example 1 was repeated with the exception that the test pieces were heated for a period of one hour at 350 degrees F. between the vapor degreasin and the application of the phosphate coating. After 16 hours exposure to 5 percent salt spray, these parts were rated as 7.

EXAMPLE 4 The procedure of Example 3 was repeated with the exception that the heating of the case hardened steel parts was carried out for a period of two hours at 350 degrees F. After 16 hours exposure to 5 percent salt spray, these parts were rated according to ASTM Method D-610-43.

While there have been described various embodiments of the invention, the methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A process for imparting improved corrosion resistance to ferrous metal surfaces which comprises case hardening the metal surface to be protected, effecting removal of oleaginous materials from the metal surface, heating the case hardened surface to a temperature within the range of about 300 to about 750 degrees F. for a period from about minutes to about 3 hours, and, thereafter, applying to the metal surface a corrosion resistant phosphate coating.

2. The method as claimed in claim 1 wherein the removal of the oleaginous materials is eifected simultaneously with the heating step by carrying out the heating at a temperature above about 550 degrees F.

3. The method as claimed in claim 1 wherein the phosphate coated ferrous metal surface is further coated with a chromate material.

4. A method for imparting improved corrosion resistance to a case hardened ferrous metal surface which comprises effecting removal of oleaginous materials from the metal surface, heating the case hardened surface to a temperature within the range of about 300 to about 750 degrees F. for a period of time from about 15 minutes to about 3 hours, and, thereafter, applying to the metal surface a corrosion resistant phosphate coating.

5. The method as claimed in claim 4 wherein the removal of the oleaginous materials is effected simultaneously with the heating step by carrying out the heating at a temperature above about 550 degrees F.

6. The method as claimed in claim 4 wherein the phosphate coated ferrous surface is given a further coating of a chromate material.

7. A corrosion resistant, phosphate coated product as produced by the method of claim 1.

8. A corrosion resitant, phosphate coated, case hardened product as produced by the method of claim 4.

References Cited UNITED STATES PATENTS 1,761,186 6/1930 Baker et al. 1486.15 2,476,345 7/1949 Zavarella 148-6.15 2,839,439 6/1958 Stapleton 148-6.15 2,875,110 2/1959 Rossander 148-6.15

OTHER REFERENCES Ebner, Steel, Oct. 3, 1949, pp. 7274, 94. Sauveur, The Metallography and Heat Treatment of Iron and Steel, 4th ed., 1935, McGraw-Hill, p. 318.

RALPH S. KENDALL, Primary Examiner. 

